1. Field of the Invention
The present invention generally relates to an engine air-fuel ratio control system. More specifically, the present invention relates to an air-fuel ratio control system configured to run the engine with a rich air-fuel ratio immediately after the engine is started and start feedback control of the air-fuel ratio afterwards such that the air-fuel ratio converge rapidly toward the stoichiometric point.
2. Background Information
Presently, many engine air-fuel ratio control systems that compute and control a fuel injection quantity of an engine. For example, Japanese Laid-Open Patent Publication No. 9-177580 and Japanese Laid-Open Patent Publication No. 10-110645 disclose engine air-fuel ratio control systems that compute and control a fuel injection quantity of an engine. These engine air-fuel ratio control systems set the air-fuel ratio to be enriched immediately after the engine is started and then gradually decreased over time such that the air-fuel ratio gradually converges toward a stoichiometric value. More specifically, a fuel injection quantity of an engine is computed and controlled using a target air-fuel ratio revising coefficient whose constituent values include a stabilization fuel quantity increasing factor that is set such that the air-fuel ratio is richened immediately after the engine is started and gradually decreased over time such that the air-fuel ratio gradually converges toward a stoichiometric value. The calculation of the stabilization fuel quantity increasing factor includes a compensation for the engine rotational speed and the load. Furthermore, an air-fuel ratio feedback revising coefficient that is set such that the air-fuel ratio converges toward a stoichiometric value based on a signal from an air-fuel ratio sensor when an air-fuel ratio feedback control condition is satisfied.
In such engine air-fuel ratio control systems, after the air-fuel ratio sensor is determined to be active, the stabilization fuel quantity increasing factor is set to 0 and the amount by which the stabilization fuel quantity increasing factor was decreased in order to reach 0 (i.e., the value of the stabilization fuel quantity increasing factor at that point in time) is added to the air-fuel ratio feedback revising coefficient, thereby increasing the value of the air-fuel ratio feedback revising coefficient. Then, an air-fuel quantity feedback control is started and an unburned fuel quantity compensating value (unburned fuel quantity balancing value) is then added to the calculation of the target air-fuel ratio revising coefficient. The unburned fuel quantity compensating value serves to ensure stability when a heavy fuel is used, and is set to make the equivalence ratio λ equal 0 when a heavy fuel is used.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved engine air-fuel ratio control system. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.